Browsing by Author "Tokunaga, Eiji"

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Development of a balanced detector with biased synchronous detection and application to near shot noise limited noise cancelling of supercontinuum pulse light
(2014-02-01)
Seto, Keisuke
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Tsukada, Toshiaki
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Okuda, Yoshinao
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Tokunaga, Eiji
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Kobayashi, Takayoshi
We report on the development of a balanced detector suited for multicolor imaging. The source pulsed light is split into probe and reference pulsed light. The reference pulse is delayed through an optical path and the probe and reference pulses are detected by a single photodetector. The signs of the detected signals of the probe and reference pulses are flipped based on a signal synchronous to the light source. Then, the signals are averaged through a low-pass filter. The output signal is proportional to the intensity difference between the probe and the reference. This balanced detector has two features: (1) both the probe and reference pulsed lights are detected by a single photodetector and (2) a voltage bias on the sign flipping compensates for the optical-intensity unbalance between the probe and reference pulsed lights. The first feature enables the probe and reference pulses to travel along a common optical path from a sample through a spectrograph to the photodetector, which minimizes the intensity unbalance between the probe and reference pulses during imaging and spectroscopy. The second feature ensures the complete balanced-detection in whole wavelength range by compensating for the optical unbalance created by deviations in the splitting ratios of the probe and reference lights at different wavelengths. Although a higher signal to noise ratio (SNR) reached to near shot noise limited SNR is attained by attaching a resonator to the photodetector for pulse repetition, the electrical bias cannot compensate for the optical balance. This unbalance is, however, corrected by adjusting the phase of the synchronous signal. We applied the present balanced detection to a stimulated Raman microscope with supercontinuum probe light and demonstrated its noise cancelling performance through capturing polystyrene beads. (C) 2014 AIP Publishing LLC.
Development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection
(2013-08-01)
Seto, Keisuke
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Okuda, Yoshinao
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Tokunaga, Eiji
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Kobayashi, Takayoshi
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電子物理學系
We report the development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection with a single light source. A white pump beam is prepared with a piece of photonic crystal fiber (PCF). The system does not require the synchronization of plural light sources or the scanning of their wavelengths, and thus a jitter-free pair of pump and Stokes beams is obtained, and a high degree of temporal synchronization is attained in the spectra. The multichannel lock-in detection (extended to 128 channels) enables the observation of pseudo-continuous stimulated Raman spectra, demonstrating the strong ability of qualitative analysis to identify various types of C-H stretching modes such as the symmetric and asymmetric modes of the methylene/methyl and aromatic groups. Images of a mixed film of polystyrene and polymethylmethacrylate are presented to demonstrate the system's spectral imaging ability. The spatial distribution of these materials is successfully captured through one-time imaging, although the noise of the white light pump beam generated with the PCF limits the system's imaging speed. (C) 2013 AIP Publishing LLC.
Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces
(2011-05-01)
Teramoto, Takahiro
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Du, Juan
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Wang, Zhuan
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Liu, Jun
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Tokunaga, Eiji
We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1', 3'-dihydro-1', 3', 3'-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2'-(2H)-indole] in a photochromic state was demonstrated. (C) 2011 Optical Society of America
Efficient Molecular Aggregation of Rhodamine 6G and Pseudoisocyanine by Light-Induced Force
(2020-05-01)
Shirakawa, Masayuki
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Kobayashi, Takayoshi
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Tokunaga, Eiji
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電子物理學系
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Department of Electrophysics
Featured Application One can generate by irradiation of light an exotic aggregation state of molecules that cannot exist in thermal equilibrium. Abstract A highly efficient light-induced aggregation of porphyrin molecules in solution was recently reported for 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS). Here, we demonstrate that rhodamine 6G (R6G) and pseudoisocyanine (PIC) also show efficient light-induced aggregation in unsaturated aqueous solution, being detected with a multichannel lock-in amplifier as the absorbance decrease/increase in the monomers/aggregates, induced by a laser at 633 nm, which is non-resonant off their main monomer absorption wavelengths. The light-induced aggregation states are H-aggregates that are hardly formed in the thermal equilibrium. The similar absorbance changes are absent in the monomer molecules fixed in polyvinyl alcohol (PVA) films. The aggregation efficiency defined as (Delta A/A)/(Utrap/kBT), where A is the absorbance of the monomers, Delta A is the absorbance increase in the aggregates, Utrap is the optical gradient force potential, and kBT is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.
Electric field-controlled dissociation and association of porphyrin J-aggregates in aqueous solution
(2011)
Nakata, Kazuaki
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Kobayashi, Takayoshi
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Tokunaga, Eiji
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電子物理學系
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Department of Electrophysics
The electrooptic effects of porphyrin J-aggregates of tetraphenyl porphyrin tetrasulfonic acid (TPPS) in aqueous solution were studied using electroabsorption (EA) spectroscopy. When the J-aggregates were three-dimensionally distributed, the EA spectra exhibited broadening in the exciton band. When a DC or AC electric field was applied for a long time, the J-aggregates with KCl were dissociated into monomers via N-mers (N = 2-4) as intermediate states, while those without KCl had an increase in aggregation. The EA spectra showed a red shift in the exciton band for N-mers, which indicates that N-mers are isolated microaggregates with a coherent aggregation number N, and isolated microaggregates have not been microscopically or spectrally observed until now. The estimated third-order nonlinear optical susceptibility chi((3)) for EA spectra in aqueous solution was 10(4) times larger than that in a polymer film. The molecular rearrangement model was applied to a variety of orientational distributions and the results were explained fairly well. The contribution of the electric double layer is the most probable reason for the large enhancement of chi((3)) for the solution sample. The dynamic equilibrium between two types of monomers, J-aggregates of various aggregation numbers and cations such as K(+) and H(+) was investigated to reveal that K(+) is more loosely bound to the constituent monomers in J-aggregates than H(+). Equilibrium equations also show that well-grown aggregates with N > 15 tend to dominate in a solution of J-aggregates, which explains why only well-developed aggregates can be observed spectroscopically.
Giant Pockels effect of polar organic solvents and water in the electric double layer on a transparent electrode
(2017-01-01)
Kanemaru, Hironori
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Yukita, Shunpei
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Namiki, Hajime
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Nosaka, Yugo
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Kobayashi, Takayoshi
The Pockels effect of polar organic solvents and water within the electric double layer (EDL) on an indium-tin-oxide (ITO) electrode is studied to find that water has the largest Pockels coefficient (230 pm V-1), followed in order by methanol (200 pm V-1), ethanol (84 pm V-1), and dimethyl sulfoxide (DMSO) (20 pm V-1). Electrolyte solutions of water and methanol have nearly the same magnitude of Pockels coefficient, while ethanol and DMSO solutions exhibit two and ten times smaller Pockels coefficients than the methanol solution, respectively. The Pockels coefficient scales well with the hydrogen-bond strength (or average cluster size) divided by the solvent viscosity. This suggests that hydrogen bonding and viscosity play crucial roles in the mechanism of the Pockels effect of these liquids.
Mechanisms of the anomalous Pockels effect in bulk water
(2018-04-01)
Yukita, Shunpei
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Suzuki, Yuto
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Shiokawa, Naoyuki
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Kobayashi, Takayoshi
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Tokunaga, Eiji
The "anomalous" Pockels effect is a phenomenon that a light beam passing between two electrodes in an aqueous electrolyte solution is deflected by an AC voltage applied between the electrodes: the deflection angle is proportional to the voltage such that the incident beam alternately changes its direction. This phenomenon, the Pockels effect in bulk water, apparently contradicts what is believed in nonlinear optics, i.e., macroscopic inversion symmetry should be broken for the second-order nonlinear optical effect to occur such as the first-order electro-optic effect, i.e., the Pockels effect. To clarify the underlying mechanism, the dependence of the effect on the electrode material is investigated to find that the Pockels coefficient with Pt electrodes is two orders of magnitude smaller than with indium tin oxide (ITO) electrodes. It is experimentally confirmed that the Pockels effect of interfacial water in the electric double layer (EDL) on these electrodes shows an electrode dependence similar to the effect in bulk water while the effects depend on the frequency of the AC voltage such that the interfacial signal decreases with frequency but the bulk signal increases with frequency up to 221 Hz. These experimental results lead to a conclusion that the beam deflection is caused by the refractive index gradient in the bulk water region, which is formed transiently by the Pockels effect of interfacial water in the EDL when an AC electric field is applied. The refractive index gradient is caused by the diffuse layer spreading into the bulk region to work as a breaking factor of inversion symmetry of bulk water due to its charge-biased ionic distribution. This mechanism does not contradict the principle of nonlinear optics.
Noise cancellation with phase-detection technique for pump-probe measurement and application to stimulated Raman imaging
(2015-05-01)
Seto, Keisuke
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Tsukada, Toshiaki
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Okuda, Yoshinao
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Tokunaga, Eiji
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Kobayashi, Takayoshi
Intensity noise on a probe beam is a serious obstacle to highly sensitive and high-speed pump-probe microscopy. In this report, a reference beam of the probe is prepared and delayed. The intensity modulation by the sample is measured as the phase modulation of the superposition of detected electrical signals of the probe and reference beams, and the intensity noise is canceled. We evaluate performance of the noise cancellation using the supercontinuum light from a piece of photonic crystal fiber, and find that the noise is canceled by similar to 26 dB. We then apply the method to a stimulated Raman microscope. This method contributes to highly sensitive and high-speed pump-probe imaging with various light sources. (C) 2015 Optical Society of America
Nonlinear Absorption Spectroscopy of Porphyrin J-aggregates in Aqueous Solution: Evidence for Control of Degree of Association by Light-Induced Force
(2017-04-15)
Shirakawa, Masayuki
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Nakata, Kazuaki
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Suzuki, Masaya
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Kobayashi, Takayoshi
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Tokunaga, Eiji
Spectroscopic evidence was obtained for molecular aggregation states to be controlled by the irradiation of light, which is off-resonant below the peak absorption energies of both monomers and well-grown J-aggregates. In low (undersaturated)-concentration aqueous solutions of porphyrin molecules (tetraphenyl porphyrin tetrasulfonic acid; TPPS) where the monomer absorbance dominates, irradiation with a 532nm laser induces a decrease in the monomer absorbance and an increase in the aggregate absorbance. The increase in the absorbance of J-aggregates occurs in a broad spectral range associated with the increase in the number of not only variously sized oligomer aggregates but also aggregates structurally different from well-grown stable J-aggregates. In high-concentration solutions where the J-aggregate absorbance dominates, a blue shift of the absorption peak of J-aggregates is induced at the same 532nm irradiation, corresponding to a decrease in the aggregation number or in the association energy. By contrast, for spincoated polymer films of monomers and J-aggregates where molecules are immobile, these features are not observed. It is remarkable that the gradient force potential is smaller by more than seven orders of magnitude than the kinetic energy of the thermal motion of the molecule at room temperature, but the absorption change in solution indicating the increase in the number of aggregates is as large as Delta A similar to 10(-3) in magnitude.
Pockels effect of water in the electric double layer at the interface between water and transparent electrode
(2007-02-01)
Tokunaga, Eiji
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Nosaka, Yugo
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Hirabayashi, Masashi
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Kobayashi, Takayoshi
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先進超快雷射研究中心
We have obtained the first experimental evidence for the Pockets effect of water, which is induced by a high electric field in the electric double layer (EDL) on the water transparent electrode interface. The electric-field induced energy shift of the visible interference fringes of a 300 nm indium-tin-oxide (ITO) electrode layer is observed, indicating a negative refractive index change at the interface. Numerical calculation reproduces well the experimental observation, showing that the signal mainly originates from water in the EDL. The Pockets constants of water are estimated to be r(33) = 5.1 x 100 pm/V and r(13) = 1.7 x 100 pm/V. The large anisotropy of the Pockets effect of water is deduced from the incidence angle dependence of the p-polarization signal. At the same time, the ITO shows a blue shift of the band gap in the UV due to the band population effect in the space charge layer. The plasma frequency in the near IR is also expected to increase due to the band population effect, since the ITO has a high doped carrier population close to metal. A negative refractive index change in the ITO space charge layer is induced from both effects, but its effect on the signal is estimated to be much smaller than that of the negative refractive index change of water in the EDL. (c) 2006 Published by Elsevier B.V.
Real-Time Vibrational Dynamics in Chlorophyll a Studied with a Few-Cycle Pulse Laser
(2011-08-17)
Du, Juan
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Teramoto, Takahiro
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Nakata, Kazuaki
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Tokunaga, Eiji
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Kobayashi, Takayoshi
We use a 6.8-fs laser as the light source for broad-band femtosecond pump-probe real-time vibrational spectroscopy to investigate both electronic relaxation and vibrational dynamics of the Q(y)-band of Chl-a at 293 K. More than 25 vibrational modes coupled to the Q(y) transition are observed. Eleven of them have been clarified predominantly due to the excited state, and six of them are concluded to be nearly exclusively resulting from the ground-state wave-packet motion. Moreover, thanks to the broad-band detection over 5000 cm(-1), the modulated signals due to the excited state vibrational coherence are observed on both sides of the 0-0 transition with equal separation. The corresponding nonlinear process has been studied using a three-level model, from which the probe wavelength dependence of the phase of the periodic modulation can be calculated. The probe wavelength dependence of the vibrational amplitude is interpreted in terms of the interaction between the "pump" or "laser," Stokes, and anti-Stokes field intermediated by the molecular vibrations. In addition, an excited state absorption peak at similar to 709 nm has been observed. To the best of our knowledge, this is the first study of broad-band real-time vibrational spectroscopy in Chl-a.
Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation
(2019-12-01)
Shirakawa, Masayuki
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Kobayashi, Takayoshi
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Tokunaga, Eiji
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電子物理學系
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Department of Electrophysics
It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole-dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.
Spectral modulation observed in Chl-a by ultrafast laser spectroscopy
(2011-11-07)
Du, Juan
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Nakata, Kazuaki
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Jiang, Yongliang
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Tokunaga, Eiji
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Kobayashi, Takayoshi
Broadband real-time dynamic vibronic coupling in Chl-a were experimentally studied using few cycle laser pulses of 6.8fs duration and a 128-channnel lock-in amplifier. Thanks to the extreme temporal resolution benefitting from the ultrashort laser pulse, the real-time modulation of the electronic transition energy induced by the molecular vibrations were calculated by the time dependent first moments of the bleaching band. The transition energy was found to be modulated periodically with the same frequencies of molecular vibration found in the Fourier amplitude spectrum of the difference absorbance real-time traces. This was interpreted to be due to the difference in the effective transition energy associated with the wavepacket motion induced by the equilibrium positions of potential curves between the ground state and the excited state. Using the values, Huang-Rhys factors for several vibrational modes involved in the spectral modulation at the room-temperature have been determined. (C)2011 Optical Society of America
Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer
(2014-08-14)
Nakata, Kazuaki
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Tokunaga, Eiji
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Du, Juan
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Xue, Bing
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Miyazaki, Jun
K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having T-c = 395 K (= and the crystal has T-c = 395 K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 cm(-1). This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.
Thermal Relaxation Spectra for Evaluating Luminescence Quantum Efficiency of CASN:Eu2+ Measured by Balanced-Detection Sagnac-Interferometer Photothermal Deflection Spectroscopy
(2020-02-01)
Chima, Hiromichi
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Shiokawa, Naoyuki
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Seto, Keisuke
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Takahashi, Kohsei
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Hirosaki, Naoto
Highly sensitive broadband photothermal spectroscopy with a white-light lamp as the excitation source was developed by combining a Sagnac interferometer and balanced detection with a photothermal deflection method. A probe beam was split by a birefringent crystal CaCO3 into signal and reference beams with a balanced intensity. This balanced detection enabled the measurement of photoexcited thermal relaxation spectra of materials in the air over the whole visible range in the weak excitation limit 50 mu W/cm(2). The photothermal excitation spectrum of Eu2+-doped CaAlSiN3 phosphors (CASN:Eu2+) with a high luminescent quantum efficiency was measured to be distinctly different from the photoluminescence excitation spectrum which reflects the absorption spectrum, revealing the thermal relaxation mechanism of the phosphor. Assuming a typical non-radiative relaxation from the higher excited states to the lowest excited state and successively to the ground state, it is demonstrated that the photoluminescence efficiency of the phosphors is readily evaluated simply by comparing the photothermal and photoluminescence excitation spectra.
Vibrational Energy Flow between Modes by Dynamic Mode Coupling in THIATS J-Aggregates
(2013-11-14)
Hasegawa, Daisuke
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Nakata, Kazuaki
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Tokunaga, Eiji
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Okamura, Kotaro
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Du, Juan
We performed ultrafast pump-probe spectroscopy of J-aggregates of 3,3'-disulfopropyl-5,5'-dichloro-9-ethyl thiacarbocyanine triethylammonium (THIATS), one of the most typical cyanine dyes, and detected excited molecular vibrations, using a sub-10 fs pulse laser. The time-resolved two-dimensional difference absorption (Delta A) spectra are observed between -314 and 1267 fs. By performing the Fourier transform and spectrogram analysis, vibrational modes in THIATS are observed at 285, 485, 555, 824, and 1633 cm(-1) and there was a modulation of the vibrational frequencies around 1633 cm(-1) which depend on the delay time, respectively. By the analysis of the modulation, energy flow is found to take place from other modes to the 1633 cm(-1) mode through the low frequency mode with similar to 50 cm(-1). Also, by fitting the real-time traces of Delta A with the sum of two exponential functions and a constant term, the average lifetimes of three electronically excited states were found to be tau(1) = 52 +/- 5 fs and tau(2) = 540 +/- 78 fs. By performing single-exponential fitting around the stationary absorption peak at 1.990 eV, in the negative time range, the electronic dephasing time, T-2(ele), is determined to be 18.30 fs.